Treating them with something alkaline — or even just with fresh water — causes them to fire, injecting you with even more venom.

That's why Dr Gershwin's nosy question about the acidity of the liquid currently in your bladder really could be a matter of life and death.

"If it's in an acidic state it works about 25 per cent as well as vinegar. Which means 25 per cent of the stinging cells will be neutralised and the rest will be discharged," she said.

"If it's in an alkaline state, it will discharge 100 per cent of the stinging cells.

"It really comes down to, are you feeling lucky?"

Tropical jelly stings

Stings in the tropics:

If you don't know for sure what gave the sting, treat with vinegar to neutralise stinging cells

If vinegar is not available, pick off remaining tentacles and rinse with seawater

Don't rinse with fresh water, as this can cause residual stinging cells to fire

A cold pack in a dry plastic bag will help with pain

If the patient has more than a localised sting or looks or feels unwell, call triple-0 and ask a lifeguard for help

Source: Australian Resuscitation Council

Dr Gershwin said rather than playing urinary roulette, folks in the tropics should just keep a bottle of vinegar in the car.

"You put vinegar in the boot of the car, it's not going to go off. And that way you don't have to think about it every time. You've just got it," she said.

"If there's something that can make a difference to your life, it's worth planning ahead for."

The vinegar won't help with the pain though — for that, follow the Australian Resuscitation Council's advice of using a cold pack or ice, as long as it's wrapped in a dry plastic bag. Remember, fresh water can cause more stinging cells to fire.

'Jellies don't sting locals' and other myths

Box jellyfish inhabit tropical Australian waters and their sting can be deadly.

Box jellyfish inhabit tropical Australian waters and their sting can be deadly.

AAP: Paul Sutherland/file photo

The peeing-on-a-jellyfish-sting myth is one of many misconceptions Dr Gershwin hears, most frequently from experienced swimmers and surfers.

"It generally comes with a dangerous mix of knowing too much and not enough. For example, people say to me things like 'I've been living here for years and I know they don't occur here'," she said.

"My favourite is somebody told me 'I swim like a turtle and they're afraid so they swim away'. I would not recommend that as a strategy."

Other myths Dr Gershwin hears are that jellies don't sting locals, or people with dark skin, or people with light skin, or people with a lot of body hair.

"None of these things are true. Jellyfish sting people with exposed skin," she said.

"If you have unprotected skin and you encounter a jellyfish you can get stung.

"Deaths are actually quite uncommon but they do occur. And the common denominator is unprotected skin."

Dr Gershwin said the best protection from getting stung was to swim between the flags on patrolled beaches, and to use a physical barrier such as a full-body lycra suit or neoprene wetsuit.

When to skip a swim

A juvenile box jellyfish in Far North Queensland. Lifeguards monitor conditions for jellyfish, so swim between the flags on patrolled beaches.

A juvenile box jellyfish in Far North Queensland. Lifeguards monitor conditions for jellyfish, so swim between the flags on patrolled beaches.

ABC Far North: Mark Rigby

On tropical beaches, Dr Gershwin said warning signs for dangerous stingers such as box jellyfish and irukandji could include:

Jellybodies on the sand: small pieces of jelly that look like crushed ice or drops of water that haven't sunk into the sand

Flat, calm weather with a light northerly breeze

Clouds of small prawns moving like a school of fish in shallow water, ideal feeding conditions for box jellyfish

Salps, which look like chains of little jellybodies. These don't sting but they co-occur with irukanji